Add assert for edge case if Thin U Requested at runtime

Arthurrequested to merge
arthurfeeney/eigen:prevent-svd-thin-runtime into master

Reference issue

Discussion in !862 (merged)
(Ignore the stuff about changing workspace sizes. That's not necessary since this case doesn't work anyway.)

What does this implement/fix?

There's an old issue when requesting thin unitaries at runtime:

 #include <Eigen/Dense>

 int main() {
     using namespace Eigen;
     using MT = Matrix<double, 5, Dynamic>;
     MT m = MT::Random(5, 4);  // fixed rows > dynamic cols

     // fails to resize stuff because U is 5x5 at compile-time, but needs to be 5x4.
     BDCSVD<MT> svd1(m, ComputeThinU | ComputeThinV);
 }

I don't think this can be "fixed," so this adds an assert for this case and updates tests to look for the assert.
This shouldn't change any behavior, basically just a fix for the tests and to improve the failure message.

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